Stabilization of photoelectric electron multipliers



July 21, 1 942.

L. SNYDER, JR 2,290,775

STABILIZATION OF PHOTOELEC'IRIC ELECTRON MULTIPLIERS Filed May 1, '19405 Sheets-Sheet l POWER SUPPLY HG. .3. M

l 1942- R. L. SNYDER, JR 2,290,775

sTAE'ILiZ'A'TIQN 0F PHOTOELEC'I'RIC ELECTRON MULTIPLIERS Filed May 1,1940 5 Sheets-Sheet 2 Po WEE COLLECTOR :S'UPPLY' 16 PHOTO 16 car/mp2 17P0 WE)? cazzsc'rori JUPPL V PHOTO cmwozw an GLOW 9 105B 18 3nnentorEzo'hard L. Snyder, J1:

July 21, 1942.- a. L. SNYDER, JR 2,290, STABILIZATION F- PHOTOELEGTRICELECT-RON MULTIPLIERS Filed Ma 1, 1940 Sheets-Sheet 3 0 2o do .120 160200 220 240 260 E k 8 -1 q 10 E R E00 400 600 600 .1000 .1200 .2'4001600 .1800 E000 Z200 Z400 VOLT/76L BETWEEN PHOTO CflV'HODE [ND .5467(ST/7GB Jnnentor Richard L.Jn yder Jr:

v ttomeg y 1942- R. L. SNYDER, JR 2,290,775

ISTAB-ILIZA'I'IGN OF PHOTOELECTRIC ELECTRON MULTIPLIERS Filed May 1,1940 Sheets-Sheet 4 m H, r m 1 y I I I j/VM 0 0 4o as 160 2'00cbzzus'cron cwswE/w' MICROHMPEHES Snventor Richard L. Snyder; Jr.

- attorney Patented July 21, 1942 STABILIZATION OF PHOTOELECTRICELECTRON MULTIPLIERS Richard L. Snyder, Jr., Glassboro, N. J., assignorto Radio Corporation of America, a corporation of Delaware ApplicationMay 1, 1940, Serial No. 332,632

12 Claims.

This invention relates to circuit arrangements for stabilizingphotoelectric electron multipliers,

and more particularly to the stabilization of such devices so that theycan be satisfactorily 'used in the photoelectric reproduction of sound.

Conventional practice in reproducing sound from motion picture films, orconverting other high quality light signals to electrical signals,employs a gas filled photocell in an effort to reduce the effects of thenoise in the coupling re sistor. Because of the critical dependence ofthe gas multiplication on the voltage between the cathode and anode,this voltage must be carefully regulated. The incandescent exciter lampused with these systems should also be operated on well regulated directcurrent because a slow change in its intensity affects the level of thesignal, and the fluctuation of the light when operated on 60-cycle A. C.causes 120-cycle' interfer ence. The regulation of the photocell voltagemay be simply accomplished by means of a glow tube, but the regulatedrectifier required by the exciter lamp is both complicated andexpensive.

Long period fluctuation of line voltage makes the elimination of theregulation of the lamp supply by the use of degenerative networks in thephotocell amplifier extremely diflicult. How-' ever, the development ofthe electron multiplier, which is equivalent to a photocell and a D.-C.amplifier, whose gain can be controlled by means of its dynodes, makes asimple degenerative system possible. When this tube is used in the placeof a photocell, with one or two glow tubes and an auxiliary photocell,the gain can be made independent of reasonable voltage fluctuations andto vary inversely with the exciting light intensity, so that themultiplier output is little affected by disturbances in the voltage orlight.

The operation of an electron multiplier depends on the phenomena ofsecondary emission. Secondary emission is caused by the impinging of astream of electrons on a secondary emissive target or dynode and, if thesurface of the dynode is properly prepared, the current emitted may bemore than six times the primary current,

the ratio-depending on the energy of the primary electrons. In aphotoelectric electron multiplier, electrons released from theilluminated photocathode are directed by positive electric fields to thefirst dynode. 'Upon striking the surface of the first dynode thephotoelectrons cause the emission-of a new and greater stream ofsecondary electrons. electric field to the second dynode, where theprocess is repeated. In this way the signal is conveyed from dynode todynode, multiplying on each impact until the final group of secondaryelectrons leaves the last dynode from which it is directed to thecollector which is coupled to the load. To provide suitable electricfields, the dynodes are peculiarly shaped and each maintained at apotential positive with respect to the photocathode, and positive withrespect to its preceding dynode( The difierence in voltage betweensuccessive dynodes is equal to that between the photocathodeand'firstdynode, so that the potential of each dynode is proportional to itsnumber.

The construction and operation of electron multipliers'is more generallydescribed, for example, in the Proceedings of the I. R. E., volume 24,for March,'1936, at pages 351 to 375, in an article by V. K. Zworykin,G. A. Morton and L. Malter, and the construction and operation of thecircular type of multiplier, which is generally referred .tohereinafter, is further described in an article entitled iTheElectrostatic Electron Multiplier, by V. K. Zworykin and J. A. Rajchman,appearing in the Proceedings of the I. R. E., vol. 27, No. 9, forSeptember, 1939, at pages. 558 to 566.

One object of the invention is to provide an improved circuit and methodof operation for a photoelectric sound reproducer which will compensatefor variations in voltage of the power supply.

Another object of the invention is to provide an improved apparatus forand method of operation of a photoelectric sound reproducer which Theseare directed by a different will compensate for variations in brightnessof the exciter lamp.

Another object of the invention is to provide an improved circuit forand method of operation of a photoelectric sound reproducer which willmaintain reproduction uniform independent of variations in power supplyvoltage or variations in exciter lamp brightness.

Another object of the invention is to provide an improved electronmultiplier circuit and method of operation which will render theoperation thereof independent of variations in power supply voltage.

Other and incidental objects of the invention will be apparent to thoseskilled in the art from the following specification and the accompanyingdrawings, in which Figure 1 is a schematic diagram of a circuit for anine-stage electron multiplier,

Figure 2 shows a modification of the circuit of Fig. 1 to compensate forvariations in the power supply voltage,

Figure 3 shows a modification of the circuit of Fig. 1 as applied to thephotoelectric reproduction of sound to compensate for variations in thebrightness of the exciter lamp,

Figure 4 shows the modification of the circuit of Fig. 1 incorporatingthe improvements of both Figs. 2 and 3 so as to compensate both forvariations in power supply voltage and for variations in exciter lampbrightness simultaneously,

I Figure 5 is a curve showing the average characteristics of one type ofelectron multiplier,

Figure 6 is a curve showing the efiect of variations of voltage of onestage in an electron mu tiplier,

Figure 7 shows a modification of the circuit of Fig. 1 involving the useof an alternating current supply and using a non-linear impedance tostabilize the output, and

V Figure 8 shows a modification of the circuit of Fig. 1 involving theuse of an alternating current supply and using a phase shifting networkto stabilize the gain.

The circuit shown in Fig. 1 is connected to a nine-stage multiplier [0.It consists of a D.-C. power supply l8 connected to the terminals of avoltage divider. All the resistors 20 are of equal value except theresistor 2| following the last dynode J at the positive end, which maybe larger to provide the voltage required by the collector l2 and load21, if the latter is of high impedance. The voltage divider should passsufiicient current to prevent the change-of current in the collectorcircuit from having an appreciable effect on the dynode voltage.

In Fig. 1, as in Figs. 2, 3 and 4, the exciter lamp of a film soundreproducer is-indicated at l5 and the optical system for directing thelight therefrom in the form of a fine line upon the sound record isindicated by the lens 16. The light from the exciter lamp I5, which isdirected upon the film I1, passes through the sound track area where itis modulated in accordance'with the sound to be reproduced and isdirected upon the photocathode. of the multiplier Ill. The emission fromthe photocathode is directed by means well known to those skilled in theart to the dynode A from which the secondary emission is directed to thesecond dynode B, this process being repeated through the successivedynodes C, D, E, F, G, H and J, and the emission from the last dynode Jis collected upon the collector l2, which is connected to the outputcircuit and power supply.

Since the ratio of secondary current to primary current of a secondaryemitting target depends on the voltage, the multiplication of a stage ina multiplier will be fixed by its potential above its source of primaryemission. Therefore, a multiplier having 1!. stages will have a gain ofR, where R is the ratio of secondary emission. Consequently, the gainwill depend on a power of the voltage.

The curve appearing in Fig. 5 shows how the ain and sensitivity of anine-stage photoelectric multiplier vary with the overall and stagevoltages. This characteristic is the average of those of a number oftubes.

It is desirable to note here that, although the gain changes rapidlywith voltage, this characteristic is essentially linear for smallsections of the curve. Therefore, for a small change in the operatingvoltage (say 1%), the gain G can be considered proportional to thevoltage V or where c is a constant in this region and Z is a constantdependent on the slope at the operating point chosen.

The curves appearing in Fig. 6 show the effect on the output or gainwhen the voltage of ijone dynode deviates from its normal voltage.'LT-he data for these curves were observed on 'a ninestage circularmultiplier operating at 100 volts per stage. The dynode voltage isreferred to that of the preceding dynode, so that 100 volts is thenormal voltage. Each of the three curves is for a different stage (thefirst, fourth, and fifth). In each case, all the stages except the oneunder observation were at their normal voltages.

It is important for the voltage stabilizingcire cult that certain partsof these characteristics are essentially linear, while, for the lightcom-- pensating circuit, other parts are hyperbolic so that G =overal1gain k =8. constant in a particular region 10 =a constant in aparticular region V'=control dynode voltage m =a constant fixed by thepotential from which V is referred h =a constant fixed by the potentialfrom which V is referred in some part G=7c (V'+m) while in others Figure2 shows the basic circuit of the first figure modified to stabilize themultiplier gain, independent of over-all voltage changes. Its operationdepends on the much greater change in multiplier gain efiected by thevoltage variation of a dynode, independent of other electrodes in thetube, than is caused by the same change in the overall voltage whenoperating on essentially linear sections of each characteristic. Theratio of the voltages which produce the same change in gain lies between6 and 10 depending on the dynode and the part of its characteristicselected. For example, the change in gain caused by a change of 8 voltsin the overall voltage can be compensated by shifting the voltage of adynode 1 volt. Therefore, if the proper part of any supply voltagefluctuation can be applied to a dynode in the proper direction, theoverall gain will not be affected.

This is accomplished by connecting one dynode G to a second voltagedivider 23, 24 which includes a source of constant voltage such as theglow tube 22 or a battery. If the second order efiects are neglected andV =the normal overall D.-C. voltage V'=the voltage between thephotocathode and the midpoint of the control dynode characteristic v=constant reference voltage E =fluctuation of V e =fluctuation ofvoltage at V on main divider e'=fluctuation of voltage at V' on controldivider G =gain of multiplier C= constant in operating region Ic=;constant in operating region a=- ,constant in operating regionZ=arbitrary constant in operating region m =arbitrary constant inoperatingregion 8 and then compensation is effected when E b V- v 71 w)1 v be a V- v and V=v(aab+ 1) It should be noted that the glow tube orbattery which supplies 2; can be replaced by any non-linear circuitelement, such as a thyrite resistor as described hereinafter inconjunction with Fig. 7. When this is done, of course, the aboveequations must be altered.

In the circuit of Figure 3, a vacuum photocell 30 is coupled to a dynodeC operated on a linear section of its control characteristic. Thephotocell 30 is exposed to the exciter lamp l5 directly by the lens 29,and passes much more current than the dynode C, while the multipliercathode ll receives only the light passing through the film. The voltagedeveloped across the coupling resistor 32 by the photocell 30 isproportional to the light on the photocell. If the dynode is operated onan hyperbolic section of its characteristic, it is possible by adjustingthis resistor 32 to cause the voltage of the dynode C to change in sucha way that the gain of the multiplier will be inversely proportional tothe light striking the photocell or that the gain times the lightintensity will be constant. Therefore, if the intensity of the excitinglight decreases, the gain of the multiplier increases in directproportion, and the lowering of the light signal striking thephotocathode is exactly compensated by the resulting increase in gain sothat the signal level remains unchanged. If the light increases, theprocess is reversed and the result is the same. The process may bedescribed as follows:

Let

G=multiplier gain I,,=multiplier collector current S,,,=cathodesensitivity of multiplier a constant L,,,=light striking multipliercathode I =photocell current S=photocell sensitivity a constant L=lightstriking photocell A= and is proportional to the film density Theconstant n is made equal to zero by the proper adjustment of the tapfrom the coupling resistor on the voltage divider 3| with normal lightintensity.

Thus the output of the multiplier is independent of the exciting lightintensity and depends only on A as long as k is a constant. But A isafiected only by the film density so that the output is proportional tothe transmission properties of the film.

The combination of a gain stabilizing circuit and an exciting lampcompensating circuit can be accomplished as shown in Figure 4. When thisis done, interaction between the circuits, which complicates theiradjustment and limits their effective operating ranges, is prevented byusing separate and widely separated dynodes C and G for the controls.This is easily done because the current in the stabilizer circuits 22,23,

24 is large enough to be negligibly influenced by that drawn by thedynode G near the collector [2, whereas the low current passed by thephotocell 30 necessitates the use of a low current stage near thephoto-cathode II for the light compensating control.

With this circuit, using a nine-stage RCA type 0-4300 multiplier,satisfactory operation was obtained with the stabilizer using theseventh dynode and the light compensator using the third dynode, asshown in Figi'4. The system, which employed two RCA type VR-l05/30l05-volt regulator tubes in series and a vacuum photocell, maintainedconstant gain for supply voltages between 750 and 800 volts and constantoutput with a 20% change in light intensity. When a 7.5- ampere 10-voltlamp, operating on alternating current was used, the use of thecompensator reduced the -cycle A.-C. component of the output due tolight fluctuations by a factor of 20 to a level within 6 db. of the shotnoise. At the same time the stabilizer reduced the efiect of a 1% A.C.ripple in the power supply by a factor of 200, making its level lessthan 6 db. above the shot noise.

This form of the invention shown in Fig. '7 is quite analogous to thatshown in Fig. 2 but the circuit is modified for the use of alternatingcurrent supply. If the frequency of the A.-C. supply is superaudible,this form of device can, of course, be used in sound reproducers but itis equally applicable to use on commercial current supplies for purposeswhere the intermittent output is not objectionable. In this arrangementinsofar as the reference numerals are the same, the parts are the sameas those shown in Figs. 1 and 2. The A.-C. supply, however, is providedthrough a transformer having the primary 40 and .the tapped secondary4|. It will be apparent that instead of the tapped secondary 4| a tappedimpedance or tapped resistance may be used, with the proper voltageapplied thereacross, and the tapped transformer in this figure as wellas in Figure 8 is shown only for the sake of simplicity. In thisarrangement the voltage divider 23, 24 is provided as in Figure 2 butthe lead 25 may be applied to the dynode F instead of G, as shown inFig. 2, depending on the manner in which the voltage is divided. Themain difference between this form of the invention and that shown inFig. 2 is that the variable impedance 42 is used instead of a glow tube.This variable impedance is of the type which varies with the voltageimpressed across it, such as a saturated iron core choke or a non-linearresistor, such as Thyrite. In order to cause a change in the voltageratio between the control dynode and its neighbors when the overallvoltage changes, such a change can be made to decrease the gain when thevoltage increases, or vice versa, or if the nonlinear impedance isproperly adjusted the gain will remain constant independent of supplyvoltage variations.

A second way of accomplishing gain stabilization on A.-C. power supplyis shown in Fig. 8 wherein a saturated iron cored inductance 43 isprovided and at an appropriate value of inductance, this is tuned toresonance by a supply frequency by means of the condenser 44. Ihe phaseof the voltage of the control dynode will then shift with respect tothat of the adjacent dynodes thereby producing a desired constancy ofoutput.

It will be apparent that in the forms of the invention shown in Figs. '7and 8 an additional gain control in accordance with average lightintensity from the light source, which affects the photocathode H, maybe applied in the same manner as described above in connection with Fig.3 as the superposition of alternating current will not interfere withthe control effect produced by the uni-directional current through thephotocell.

I claim as my invention:

1. In a sound reproducer, a light source, means for modulating lightfrom the source in accordance with sounds to be reproduced, a photoelectric electron multiplier responsive to said modulated light, saidmultiplier including a photoelectric cathode, a plurality of multiplyingelectrodes and a collecting electrode connected to an output circuit, asource of power supply connected to all of the electrodes of saidmultiplier, and a photocell in the path of unmodulated light from saidsource and connected to said power supply and one of said multiplyingelectrodes 'for rendering the output of said device independent ofvariations in the illumination from said light source the saidelectrodes being connected to said power supply through a voltagedividing network, one of the electrodes being connected to a separatevoltage divider from the others and including a source of constantpotential in said separate voltage divider for maintaining the output ofsaid multiplier constant irrespective of variations in said powersupply.

2. In a sound reproducer, a light source, means for modulating lightfrom th source in accordance with sounds to be reproduced, aphotoelectric electron multiplier responsive to said modulated light andprovided with a photoelectric cathode, a plurality of multiplyingelectrodes and a collecting electrode connected to an output circuit, asource of power supply connected to the electrodes of said multiplier,and a photocell responsive to unmodulated light from said source andconnected to said power supply and one of said multiplying electrodesfor maintaining the output of said device constant irrespective ofvariations in the illumination from said light source.

3. In combination with an electron multiplier having'a cathode, acollector and a plurality of intermediate dynodes, a source of potentialconnected to said collector, said cathode and said dynodes, one of saiddynodes being connected to said source of potential through a voltagedividing circuit including a variable impedance and the other of saidelectrodes being connected to said source of potential through separatepotential-dividing means.

4. In combination with an electron multiplier having electrodesincluding a cathode, a collector and a plurality of dynodes between saidcathode and collector, a source of potential for said -'electrodes, avoltage dividing circuit including a variable impedance connecting on ofsaid dynodes to said source of potential and separate voltage dividingmeans connecting the remainder of said electrodes to said source ofpotential.

5. Apparatus as defined in claim 4 wherein the variable impedance is aglow tube.

6. Apparatus as defined in claim 4 wherein the variable impedance is abattery.

'7. Apparatus as defined in claim 4 wherein the variable impedance is asaturated reactor.

8. Apparatus as defined in claim 4 wherein the variable impedance is asaturated reactor tuned to resonance by a capacitor.

9. Apparatus as defined in claim 4 wherein the variable impedance is aresistor the resistance of which varies as the current therethroughchanges.

10. Apparatus as defined in claim 4 wherein the variable impedance is aresistor the resistance of which changes as the current therethroughchanges and having a capacitor connected in shunt therewith.

11. In combination, a light source, means for modulating light from thesource, a photoelectric electron multiplier responsive to said modulatedlight, said multiplier including a photoelectric cathode, a plurality ofmultiplying electrodes and a collecting electrode connected to an outputcircuit, a source of power supply connected to all of the electrodes ofsaid multiplier, and a photocell in the path of unmodulated light fromsaid source and connected to said power supply and one of saidmultiplying electrodes for rendering the output of said deviceindependent of variations in the illumination from said light source,

the said electrodes being connected to said power supply through avoltage dividing network, one of the electrodes being connected to aseparate voltage divider from the others and including a source ofconstant potential in said separate voltage divider for maintaining theoutput of said multiplier constant irrespective of variations in saidpower supply.

12. In combination, a light source, means for modulating light from thesource, a photoelectric electron multiplier responsive to said modulatedlight and provided with a photoelectric cathode, a plurality ofmultiplying electrodes and a collecting electrode connected to an outputcircuit, a source of power supply connected to the electrodes of saidmultiplier, and a photocell responsive to unmodulated light from saidsource and connected to said power supply and one of said multiplyingelectrodes for maintaining the output of said device constantirrespective of variations in the illumination from said light 10source.

RICHARD L. SNYDER, JR.

